Some sensor cells have sensor fluids that have a desired amount of the sensor fluid in a vapor phase during operation. The sensor cell provides a signal path through which a signal, such as an electromagnetic signal or an acoustic signal, may pass. For instance, a signal can be introduced into and/or received from the sensor cell during operation of the sensor cell and by interacting with the sensor fluid in the vapor phase. The sensor cell is typically heated during operation to provide the desired amount of the sensor fluid in the vapor phase. During inoperative periods, the cell cools, and a large portion of the sensor fluid in the vapor phase condenses into a condensed phase, that is, a liquid and/or a solid phase. Examples of such sensor cells are atomic clocks and magnetometers, which use alkali metals as sensor fluids, and have optical signal paths in the sensor cells. There is great interest in reducing the size and power requirements of such sensor cells, to enable widespread use in low cost, handheld, mobile, and other applications. As the sizes of the sensor cells are reduced, a problem arises in which the sensor fluid in the condensed phase blocks the signal path in the sensor cell. In cases where some of the sensor fluid remains in the condensed phase during operation, for example to ensure attaining the required amount of sensor fluid in the vapor phase, the sensor fluid in the condensed phase undesirably interferes with the signal during operation. In some cases, less sensor fluid is disposed in the sensor cell so as to minimize condensation in the signal path, resulting in substantially all the sensor fluid being in the vapor phase during operation. In these cases, the vapor pressure of the sensor fluid, and hence the amount of sensor fluid in the sensor cell may vary from sensor cell to sensor cell, thereby introducing variation in performance uniformity of the sensor cells.